It is known that launch vehicles of payloads such as satellites require measures to protect the payload from the sound fields generated by lift-off and flight. It is also known that as new high energy launch vehicles are utilized the increased total lift-off thrust of these vehicles produces higher acoustic field intensities than current launchers. Therefore, conventional measures for protection of payload are inadequate.
A fairing is the shroud or cover that surrounds the payload or cargo which will be launched for use. Generally, the cargo is a satellite although any other instrument or vehicle may be launched. The fairing protects the payload from the atmosphere. When the launch vehicle is above atmosphere the fairing is jettisoned and the payload is released when the propulsion operations are completed. Currently, fairings are generally constructed with a solid metal wall or a layered wall including a simple, one dimensional aluminum honeycomb core with composite face sheets. These "bare wall" fairings do not adequately protect some payloads from damage. The robustness of the payload varies. Some payloads are more fragile and sensitive than others. These payloads require more protection.
The payload must be protected from the high temperatures, large forces, and vibrations due to noise encountered during launch. The acoustic energy produced can damage the payload. The acoustic energy can reach 150 dB which produces vibrations that can lead to mechanical damage. In the case of satellites, appendages or antennas can break. Circuit boards can be damaged and mirrors or lenses can be damaged or moved out of alignment. Any resulting damage can seriously hurt or totally impair the capabilities of the satellite.
Acoustic blankets have been used to absorb the acoustic energy to protect the payload. The blankets are attached to the inside surface of the fairing wall. These blankets are usually made of fiberglass batting or a combination of fiberglass sheets and batting which are of different thicknesses and are layered together. The use of blankets has been found to be most effective at frequencies above 300 Hz but do not provide enough protection below about 100 Hz. For frequencies of greater than 300 Hz, the wavelengths are fairly short and the acoustic energy can be fairly easily absorbed by the blankets. However, for frequencies below 100 Hz, the wavelengths are fairly long and there is not a sufficient depth of material in blankets to absorb a significant portion of the energy. Therefore, the blankets are not very effective at these frequencies. In the past, to increase effectiveness of the blankets, the thickness has been increased to provide more absorption. As the thickness of the blankets is increased, the cost, volume, and weight of the blanket is increased. Of course, an increase in weight and volume of the blanket decreases the weight and volume available for payload. Thus, the launch vehicle payload lift capability is adversely affected by use of thick, heavy blankets.
Two other approaches to reducing acoustic transmission through the fairing wall have been taken. The stiffness of the wall has been increased and the mass per unit area of the wall has been increased. Neither of these approaches are considered desirable because they usually increase fairing weight, which reduces payload weight delivered to orbit.
Consequently, a need remains for a payload fairing which can suppress the acoustic energy generated by the launch of the payload to protect the payload without penalizing vehicle lift capability. The invention provides high acoustic attenuation over a wide frequency band including low frequencies.